Acoustic imaging

ABSTRACT

Single hand supportable and operable apparatus for providing an output signal characteristic of the morphology of a respiratory tract includes an acoustic pipe for exchanging acoustical energy with the tract. The pipe has an open first end in communication with an opening in the respiratory tract. A transducer, such as a loudspeaker, is coupled to the pipe for launching acoustical energy into the pipe, producing an incident wave towards the opening in the tract and a reflected wave to form a transient wave field in the pipe representative of the morphology of the tract. Preferably, first and second pressure wave sensing transducers, such as microphones, mounted along the length of the pipe in spaced relationship provide first and second transduced signals representative of the transient wave field. A processor processes the first and second transduced signals to provide an output signal characteristic of the morphology of the tract, such as the cross-sectional area as a function of the distance from the opening in the tract.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.08/117,176 filed on Jun. 16, 1993 and now U.S. Pat. No. 5,666,960, whichwas a Continuation-In-Part of U.S. patent application Ser. No.07/808,907 filed on Dec. 17, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to acoustic imaging of mammalian airwaymorphology and more particularly concerns noninvasively obtaining asignal representative of the cross-sectional area of an airway (e.g.,oral, nasal, or pulmonary) of a subject (e.g., a person or an animal)using electroacoustical transducers.

2. Brief Description of Related Art

A one-dimensional image of the cross-sectional area of an airway as afunction of axial position along the airway may be determined fromacoustic reflections measured by a single electro-acoustic transducerplaced in a position remote from the airway opening. This image isreferred to as an area-distance function and is represented by A(x)where x is the axial position along the airway.

Knowledge of the area-distance function, A(x), is useful for example inthe diagnosis of mammalian pathologies associated with oral airways,larynx, pulmonary airways, and nasal airways. These pathologies includebut are not limited to obstructive sleep apnea, asthma, obstructivepulmonary disease, tracheal stenosis, and nasal septum deviation.

Accurate information about the area-distance function is also useful inthe study of airway growth and its disruption and sequelae ofbronchopulmonary dysplasia in children.

One approach to using a single electro-acoustic transducer in acousticimaging is described in U.S. Pat. No. 4,326,416 granted Apr. 27, 1982,to Jeffrey J. Fredberg entitled ACOUSTIC PULSE RESPONSE MEASURING. Inall of the single-transducer approaches described previously, a hiddenconstraint pertains. The associated theories assume implicitly that oncepropagating to the left within the apparatus, acoustic returns encounterno reflection sites within the wave-tube apparatus itself, which isassumed to be a reflectionless transmission line; there must be nosecondary rightward travelling waves and no acoustic reverberationwithin the wave tube. However, since the loudspeaker is an unavoidableand major reflection site for the wave travelling to the left, thedistance separating the loudspeaker from the receiving transducer mustbe greater than the maximum airway penetration depth of interest; thisensures that secondary reflections from the speaker arrive at thereceiving transducer only after data acquisition has been completed. Asa result of this distance constraint, imaging instruments for airwayimaging previously described in the literature are 1 to 2 meters or morein length (Brooks et al., Reproducibility and Accuracy of Airway Area byAcoustic Reflection, J. Appl. Physiol.: Respirat. Environ. ExercisePhysiol. 57(3): 777-787, 1984; D'Urzo et al., Effect of CO₂Concentrations on Acoustic Inferences of Airway Area, J. Appl. Physiol.60:398-401, 1986); and some are as long as 5 meters (Fredberg J. J. etal., Airway Area From Acoustic Reflections Measured at the Mouth, J.Appl. Physiol.: Respirat. Environ. Exercise. Physiol. 48(5): 749-758,1980).

A two-transducer approach is described in a paper of M. R. Schroederentitled "Determination of the Geometry of the Human Vocal Tract byAcoustic Measurements" in J. Acoust. Soc Am. 41(4), 1002-10 (1967).However, as with the single-microphone approach, Schroeder'stwo-microphone method was never embodied successfully into a small,compact, hand holdable light-weight working apparatus and in fact neverachieved airway reconstructions from human or animal airways.

The present invention is based upon a new two-transducer method and anew associated theory that permits practical application of airwayreconstructions by acoustic reflections. Because this theory explicitlyincorporates reverberation within the wave tube and does not demandnon-overlapping time windows of incident and reflected waves, it removesthe distance constraint described above, permitting placement of theloudspeaker or launching transducer close to the receiving transducers.

As such, the new theory allows fabrication of a practical miniatureapparatus whose overall length is only a few centimeters rather thanmeters. The apparatus can image the respiratory tract of mammaliansubjects, including the nasal oral and pulmonary cavities.

SUMMARY OF THE INVENTION

In an assembly for acoustically imaging the internal morphology ofportions of the respiratory tract of a mammal, including a human, theimprovement which comprises a light-weight, easy to manipulate,hand-held acoustic imaging head which is rugged and entirely handsupportable and operable by an operator, throughout an imagingprocedure, which head comprises;

A. a rugged hand-holdable housing having

1. an elongate body, defined by

(a) a top end;

(b) a base end;

(c) an outer wall extending between the top end and the base end; and

(d) an internal chamber;

2. an aperture through the housing top end, providing fluidcommunication between the internal chamber and the outside of thehousing; and

3. a shape and configuration of the outer wall facilitating gripping ofthe housing with a human hand;

B. an acoustic pipe for transmitting acoustical energy and receiving thereflected acoustical energy, mounted in the aperture, said pipe having afirst end within the chamber and an open second end outside of thehousing, said second end of the acoustic pipe being adapted forconnection of the acoustic pipe to an orifice leading into therespiratory tract;

C. a launching transducer mounted in the chamber and coupled to thefirst end of the acoustic tube, for launching acoustical energy into theacoustic pipe, propagating an incident wave out of the open second end;

D. at least one acoustic pressure wave sensing transducer mounted on theacoustic pipe at a location between the first and second ends of theacoustic pipe, for sensing reflections of the incident wave, receivedback in the acoustic tube through the open second end and generating asignal; and

E. means at least partially within the chamber, connected to theacoustic wave sensing transducer, for transmission of signalstransduced, to processor means for processing said signals into aprocessor output signal characteristic of the morphology of a sitewithin the mammal's respiratory tract.

The invention provides signals representative of airway morphology inapparatus that is relatively small and portable. The invention may beused for diagnostic and screening purposes in a confined area such as alaboratory, a doctor's office, a place of work, and at bedside.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation of a preferred embodimenthead of the invention.

FIG. 2 is a combined block-pictorial diagram illustrating operation ofthe head shown in FIG. 1.

FIG. 3 is a fragmentary view of the outside wall of the housing for theembodiment head of the invention of FIG. 1, showing a representativecontrol panel.

FIG. 4 is an electrical wiring diagram for he embodiment head shown inFIG. 1.

FIG. 5 is a partially fragmented sideview of a disposable nasal couplingdevice of the invention used in conjunction with the head of FIG. 1.

FIG. 6 is a view from above of the coupling device shown in FIG. 5.

FIG. 7 is an alternative shape for the coupling opening in the device ofFIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Those skilled in the art will gain an appreciation of the invention froma reading of the following description of the preferred embodiments whenviewed with the accompanying drawings of FIGS. 1-7, inclusive.

Referring first to FIG. 1, a cross-sectional side elevation, there isseen a preferred embodiment acoustic imaging head 10 of the inventionfor imaging the internal morphology of portions of the respiratory tractof a mammal, 20 including a human. Head 10 comprises a hand-holdablehousing 12 having an elongate body 14 defined by a top end 16 and a baseend 18 which presents a planar surface 19 for standing the head 10 on aflat surface in an upright position. The housing 12 presents an outerwall 20 extending between ends 16, 18 and defining an internal, closedchamber 22. Wall 20 advantageously includes integrally molded fingerholds 24 and a palm grasping indentation 26. The outside surface of wall20 may be a frictional surface to facilitate holding and operating thehead 10 by a single human hand. The overall weight of head 10 will besuch that it is light and easy to operate and manipulate. The head 10 isfully supportable by hand-holding by the operator. An aperture 28pierces end 16 of the housing 12. The aperture 28 has mounted thereinend 34 of an acoustic pipe 30 having a closed first end 32 withinchamber 22 and an open end 34 outside of the housing 12. The end 34 ofacoustic pipe 30 includes a rib 36 encircling the circumference of end34, which functions as a means for coupling to the acoustic pipe 30 anadaptor in the form of coupling device 80 for mating with an orifice ofthe mammal respiratory tract for imaging. The acoustic pipe 30substantially traverses chamber 22 and its dimensions dictate theoverall dimensions of the housing 12. For example, the acoustic pipe 30is advantageously about 2.0 to 4.0 cm, preferably about 1.2 cm indiameter and has a length of between about 5 to about 40 cm, mostpreferably 5 to 20 cm. The end 32 of pipe 30 is closed by mountingthereon a loudspeaker or launching transducer 38 which, uponenergization, will launch acoustic energy into the interior of acousticpipe 30, propagating an incident sonic wave towards open end 34 of pipe30 and outside the end 34. When the head 10 is coupled to an orifice inthe respiratory tract for imaging through coupling device 80, thepropagated sound wave will enter the respiratory tract, strikeanatomical features in the tract and be reflected back through end 34into the interior of acoustic pipe 30 to form a transient wave fieldwithin the pipe 30. This wave field is representative of the morphologyof the respiratory tract. Two spaced apart pressure transducers 40, 42(such as Endevco series 8510 B microphones) are mounted on the acousticpipe 30 with their pressure sensor flush with the inner walls ofacoustic pipe 30 in order to reduce parasitic acoustic reflections. Thetransducers 40, 42 are advantageously separated from each other adistance of from about 1.0 to about 15 cm. and both are separated fromthe end 34 of acoustic pipe 30 by at least about 2.0 cm. When the head10 is to be used only for the imaging of relatively shallow cavities,i.e.; interior respiratory tract cavities which are to be imaged andwhich are close to the transducer (at most about 2 to 3 cm., such as thenasal vestibule, only one of the transducers 40, 42 need be present oractive. However, when only one of the transducers 40, 42 is employed tosense reflected acoustical waves it is necessary to calibrate the head10 for each imaging as will be described more fully below. The launchingtransducer 38 is energized to transmit a one-dimensional, essentiallylossless acoustic wave into the acoustic pipe 30 by an electrical signaltransmitted via conductor 48 which may terminate externally in aplug-type connector 50 mounted on the housing 12 wall 14 and in anamplifier 52 which will be described more fully hereafter. Thetransducers 40, 42 are each electrically connected by separateelectrical conductors 54, 56, respectively, which also can terminate inplug-type connectors 58, 59 mounted on the housing 12 wall 14. Theconnectors 58, 59 can be used for connection to signal processing meansexternal of head 10, as will be discussed more fully hereinafter.

In one embodiment head 10 of the invention, the connectors 50, 58, 59are connectable through a communications cable to a signal processor,including a host computer (not shown in FIG. 1) which is remote from thehead 10. In this embodiment, the mode of the operation will be describedwith reference to FIG. 2 as follows. FIG. 2 is a combinedblock-pictorial diagram illustrating the operation of an imaging systemusing the head 10 of the invention.

Referring to FIG. 2, a simplified assembly includes as the launchingtransducer 38 a loudspeaker such as model MDR 434. Acoustic pipe 30 isof length L. First receiving transducer 40 is located at x=o where x isthe axial position along pipe 30, second receiving transducer 42 islocated at x=L, adjacent the airway (e.g. an oral, nasal, or pulmonaryairway) of a subject mammal. Transducer 38 launches a one-dimensionalessentially lossless acoustic wave into pipe 30, towards x=L. Thelaunched incident wave travels through pipe 30 and into the airway. Areflected wave, or echo, representative of gradients in the acousticimpedance in the airway then propagates back into pipe 30 towardslaunching transducer 38. Receiving transducers 40 and 42 individuallyand together sense the pressure waves associated with the echo andprovide transduced electrical signals representative of the incident andreflected waves for processing by microcomputer 60.

Microcomputer 60 generates a digital probe signal converted to an analogsignal by D/A converter 62, and amplified by amplifier 52 to drivelaunching transducer 38 and launch the incident probe acoustic wave. Thetransduced outputs from receiving transducers 40 and 42 are band-passfiltered by preamplifier 66 and then converted by A/D convertor 68 intodigital signals. Microcomputer 60 stores these digital signals in itsRAM capability.

Microcomputer 60 processes the stored digital signals to provide anoutput signal A(x), i.e., a one-dimensional image of the cross-sectionalarea of the airway as a function of axial position, x, along the airway.Computer 60 preferably processes these signals in accordance with theWare-Aki algorithm ("Continuous and Discrete Inverse Scattering Problemsin a Stratified Elastic Medium. I. Plane Waves at Normal Incidence", J.Acoust. Soc. Am., 54, 4, 911-921, 1969) to provide the area distancefunction, A(x), from the impulse response of the airway, h(t). Therelationship between the pressure field and h(t) may be derived asfollows. The pressure field within the tube in the domain 0≦x≧L can bedescribed as the superposition of two one-dimensional waves propagatingwith the same wave speed but in opposite directions as given by

    p(x,t)=p.sub.r (x,t)+p.sub.l (x,t)                         (1)

where t is time, p_(r) is the incident wave propagating to the right(i.e., from x=o towards x=L), and p_(l) is the reflected wavepropagating to the left (i.e., from x=L towards x=o).

The pressure conditions at x=o and x=L are given by

    p.sub.l (L,t)=p.sub.r (L,t)*h(t)                           (2)

and

    p.sub.r (O,t)=p.sub.l (O,t)*s(t)                           (3)

where s (t) is the impulse response of the loudspeaker and * denotes theconvolution operation. Given that a one-way propagation delay is τ=L/vwhere v is the velocity of sound, the following relationships exist:

    p.sub.l (O,t)=p.sub.l (L,t-r)                              (4)

    p.sub.r (L,t)=p.sub.r (o,t-r).                             (5)

Equations (1) through (5) may be combined by mathematical techniqueswell-known in the art (e.g., the Fourier transform, and algebra) toyield

    h(t)*{p(O,t)-p(L,t-τ)}=p(L,t+τ)-p(o,t)             (6)

and

    h(t)*s(t)=δ(t-2r).                                   (7)

Equation (7) indicates that both waves propagate with an equal, non-zerodelay. In equation (7), the symbol δ denotes the well-known impulsefunction which is sometimes called the delta function. Equation (6)identifies the relationship between the pressure field and h(t).Equation (6) may be discretized by the Riemann sum approximation toyield

    h(nΔt)={1/p(o,o)}{p(L,(m+n)Δt)-p(o,nΔt)}-k=n(8)

    {Σh((n-k)Δt)/p(o,o)}{p(o,kΔt)-p(L,(k-m)Δt)}k=1

where Δt is the sampling duration of the time discretization, n is theset of integers 1,2,3, etc., m is an integer such that τ=mΔt, p(o,o)denotes the first non-zero pressure value at x=o, k is the index of thesummation, and the lower and upper limits of summation are,respectively, k=1 and k=n.

In brief summary, microcomputer 60 processes the stored digital datasignals representative of the transduced signals from thespaced-transducers 40,42 to provide a signal representative of theimpulse response of the airway, h(t), according to equation (8).Microcomputer 60 then processes the signal h(t) in accordance with theWare-Aki algorithm to provide a signal, A(x), representative of themorphology of the airway. The signal A(X) can then be stored in thestorage capability of microcomputer 60 for future call-up whencomparisons are desired, to subsequent images.

The wave propagation in pipe 30 may be assumed lossless. The earlyportions of the transduced pressures at x=o and x=L are then identicalexcept for the propagation delay t. Microcomputer 60 may determine thepropagation delay by minimizing mean square differences between thetransduced signals in the early part of their respective transients. Therelative gain of the transducers may be determined in a similar manner.To obtain sufficient time resolution, microcomputer 60 preferablyinterpolates the transduced signals to achieve an effective samplingperiod of 0.75 μs (i.e., Δt=o0.75 μs).

Equation (8) requires preferably the propagation delay to be an integralmultiple of the sampling period t, i.e. τ=mΔt. Microcomputer 60preferably interpolates and resamples digitized transduced signals suchthat the propagation delay corresponds to 24 time steps, i.e., τ=24Δt.This value of the propagation delay corresponds to a spatial stepincrement of about 0.2 cm.

In providing h(Δt) in accordance with the equation (8), the firstnon-zero pressure value p(o,o) is preferably larger than some minimalthreshold value to maintain stability. The pressure values occurringbefore this first threshold pressure are initially neglected to obtain afirst approximation of h(t). To deemphasize errors that may beintroduced by the threshold it is advantageous to provide a correctedh(Δt) characterized by increased stability and accuracy by convolvingthe first approximation of h(t) with the digitized pressure valuesoccurring prior to the first non-zero pressure (i.e., the pressurevalues that were initially neglected). Microcomputer 60 then preferablybandpasses the discrete values sequence h(nΔt) that represents theimpulse response of the airway h(t) with a digital, linear-phase, finiteimpulse response (FIR) filter having a passband from 0.01 kHz to 9 kHzto attenuate physiologic noise associated with airway wall non-rigidity,instability of the impulse response, h(Δt), and artifacts associatedwith acoustic cross-modes.

Microcomputer 60 then processes the corrected h(Δt) signal in accordancewith the Ware-Aki algorithm to provide an output signal representativeof the area-distance function, A(x), of the airway graphicallyrepresented.

The microcomputer 60 (such as Compuadd model 320 with an Intel 80386microprocessor operating at 20 MHz) is coupled to a converter modulehaving a 12-bit analog-to-digital (A/D) converter 68 with a samplingperiod typically of 24.0 μs and a 12 bit digital-to-analog (D/A)converter 62 coupled to pre-amplifier 66 typically with a band-passfilter having a passband from 0.1 kHz to 10.0 kHz (such as Tektronixmodel AM 502) that is coupled to transducers 40 and 42. D/A converter 62is coupled to transducer 38 through amplifier 52.

As mentioned above, when the head 10 of the invention is to be used forimaging relatively shallow body cavities near the orifice for couplingto the head 10, for example, the nasal vestibule to a depth of about 1to 3 cm., one can use a head 10 where only one of the transducers 40, 42is present or active. In this case, the relationship between thepressure field and h(t) may be derived as described in the U.S. Pat. No.4,326,416 which is incorporated herein by reference thereto. As alsomentioned above, with only one operative receiving or sensingtransducer, it is necessary to calibrate the instrument. Thiscalibration procedure is also described fully in the U.S. Pat. No.4,326,416. An advantage of the preferred two transducer (microphone)apparatus of the invention is the dispensation with of the calibrationprocedure requirement.

The microcomputer 60 may also be coupled electronically to the inputterminals of a display module (not shown in FIG. 1 or 2) for visualdisplay of processed signals from the transducers 40, 42. Alternativelyor in addition, the processed signals can be input to a conventionalprinter for a printed record of the imaging results. Simultaneous visualdisplay of processed signals is advantageous to the operator and permitsthe operator to practice a preferred method of operation. It will beappreciated by those skilled in the art that a hand operated device foruse in imaging, for example, the nasal passages of a mammal requires asteady operator's hand and patient-subject to obtain consistently goodand reproducible signal data. Slight inopportune movement on the part ofthe operator or patient can generate faulty data. The operator withexperience can see on a visual display, inconsistent signals and periodsof time when a series of incident waves propagated by the head arereflected, sensed and processed as consistent signals representative ofthe desired image. The operator in fact may have to manipulate the head10 into certain positions in order to obtain consistent and reproduciblesignals (for example when a good coupling seal is difficult to achievebetween the orifice and head 10). To overcome this problem, the head 10is advantageously used in a "pre-trigger" mode to sort out good signalsfrom faulty signals, i.e.; inconsistent signals. This is done by use ofa "circular buffer" whereby the microprocessor 60 is programmed to onlystore in the RAM the last 10 of 10 plus X signals received from A/Dconverter 68 (first in, first out). These last 10 signals can representa collection time period of about 2 seconds. When the operator views thesignals processed and put out by the microprocessor 60, as consistent,the operator can stop acquisition of new signals to maintain the last 10consistent signals in the microprocessor memory for storage and futurecall-up. In this way, the useful signals are stored for later recall.

In a most preferred embodiment head 10 of the invention, electricalpower and/or communication cables between the head 10 and the signalprocessing components of the imaging system described above areeliminated. The imaging system will then be self-contained in the singlehand-held device of the invention. Referring again to FIG. 1, miniaturesignal processing means 100 is mounted in the chamber 22 and includesthe pre-amplifier 66, converter D/A 62 and converter A/D 68 withmicrocomputer 60. Integration of the computer circuitry into from one tothree chips enables this miniaturization. A rechargeable battery 102such as an AAA size battery is advantageously mounted in chamber 22proximal to the housing base end 18 and includes a socket 104 forelectrical recharge by connection to AC wall outlet sources. The batterypack 102 is electrically connected to the processing means 100 and thelaunching transducer 38, as a power source, controlled by switchcircuitry described below (wiring not shown in FIG. 1; see FIG. 4). Theconnectors 101, 103 are for connection of means 100 to a visual displaymonitor for observation and/or to a printer for obtaining a printedrecord of the processed signals.

Referring to FIG. 3, a portion of the outside of housing 12 wall 14 isshown, with a control 108 for energization and operation of the head 10,located in one of the fingerholds 24 to facilitate one-handed operationof head 10. For the sake of brevity, a switch 109 is located in anadjacent fingerhold 24, which may be used to control such functions asSTART ACQUISITION OF DATA; STOP ACQUISITION OF DATA; DISPLAY DATA;SELECT DATA FOR DISPLAY; STORE DATA; RECALL DATA FOR DISPLAY; COMPARENEW DATA TO RECALLED DATA, etc. Any and all of these functions (andothers) can be controlled by finger-operated switches. The electricalwiring necessary for these control functions are not shown in FIG. 1,for clarity of the drawing, but will be conventional and realized bythose skilled in the art of electrical wiring.

FIG. 4 is a wiring diagram for the preferred embodiment head 10described above, which is completely self-contained.

Referring to FIG. 5 there is shown in a partially fragmented side view,a sterilizible, disposable nasal coupling device 80 having an input end82 that attaches to the output end of pipe 30 and an output end 84 forinsertion into a nostril. Channel 85 on the inside wall of couplingdevice 80 mates with and receives the rib 36 on end 34 of tube 30, forsecure, removable attachment. Channel 85 and rib 36 cooperate to providea means for attaching the acoustic pipe 30 end 34 to the coupling device80. Other means of attachment will be apparent to those skilled in theart, such as the so-called "bayonet mount", screws, frictional fits,male-female connectors and the like. Nasal coupling device 80 may haveoutput ends 84 of different sizes for snug sealing engagement with theinside of nostrils of different sizes. Its internal area contour is suchthat impedance matching between subject and apparatus is maintained withmaximum acoustic energy transmission.

FIG. 6 is a top view of the coupling device 80 shown in FIG. 5 and showsthat the rim or edge 94 of end 84 may be relatively thin, i.e.; on theorder of about 0.5 to about 1.5 mm in uniform thickness for a distanceof about 10 mm from the edge 94 toward end 82 to facilitate acomfortable fit within the nostril and to allow trimming circa 10 mm offthe length of the device 80. The wall 87 thickness below the 10 mmdistance from the edge 94 may be increased towards end 82, where morestructural stability and less flexibility is desired. The wall 87 of theoval shaped device 80 (as seen from above in FIG. 6) tapers from end 82inwardly to end 84, on an inward slope of about 10° to 15° from theperpendicular. Similarly, there may be disposable coupling devices withoutput ends 84 of different sizes for snugly coupling to other airwayorifices of different sizes, including the mouth.

The preferred coupling device 80 may be fabricated from a wide varietyof materials, both flexible, and inflexible in character. Representativeof such materials are a wide range of synthetic and natural polymericresins, preferably having properties which do not inhibit passage ofacoustic waves through the device 80 from and to the pipe 30. Thus, thedevice 80 is advantageously fabricated from polycarbonate resins,polyvinyl chloride, polyvinyl alcohol, ABS resins, polystyrene,polyurethane, polytetrafluoroethylene, natural rubber and like resins. Apreferred material is a flexible polysilicone rubber, for sealingengagement with an orifice to be used. Indicia such as line 86 can beplaced on the device 80 to indicate the depth of desired insertion intothe nostril which is preferably 2 to 3 mm penetration at most. This isadvantageous to insure accurate imaging. Also, scribe marks 88 can beplaced on the device 80, indicating where the device 80 can be cut totrim it for a larger sized nostril. A vertical rib or mark on the axialline of device 80 may be placed on the outside body of device 80 as areference mark or indicia to facilitate placing the device 80 inposition on a patient's nose, removing it and replacing it in the sameposition for subsequent imaging. Note that the device 80 can also befitted internally with a filter 90 which does not inhibit passage ofsound waves, but will stop passage of fluids such as mucous.

The opening of device 80 at end 84 as shown in FIG. 6 is round (oval) asdefined by the wall 87. The shape of the opening may be circular, of anyshape including perfectly round, oblong or shaped like a race-trackhaving a greater dimension in one direction. FIG. 7 shows a preferredcircular shape for the opening of a device 80 at end 84. The geometry ofthe end 84 of device 80 preferably matches the geometry of the orificeto be coupled without any substantial distortion of the nasal vestibule.

The invention is not to be limited to the description of the preferredembodiment described above, but include within the spirit and scope ofthe invention other embodiments.

For example, pipe 30 (FIG. 1) could be 1.2 cm in diameter and 10 cm inlength. In this case, the two transducers are preferably separated by3.0 cm but transducer 18 is still located 2.0 cm from the airwayopening. With the 10 cm length tube, the propagation delay, τ, is madeto correspond to seven time steps, i.e.; τ=7Δt. This value of thepropagation delay corresponds to a spatial step increment of about 0.2cm.

Also, an algorithm other than the Ware-Aki algorithm could be used touniquely determine the area-distance function A(x), of the airway fromh(t). Also, algorithms other than the equation (8) above can be used todetermine h(t) from the pressure field.

What is claimed:
 1. A coupler for coupling a head assembly of anacoustic imaging device to the orifice leading to the respiratory tractof a mammal for acoustic imaging, which head assembly comprises;A. arugged hand-holdable housing having1. an elongate body, defined by(a) atop end; (b) a base end; (c) an outer wall extending between the top endand the base end; and (d) an internal chamber;
 2. an aperture throughthe housing top end, providing fluid communication between the internalchamber and the outside of the housing; and
 3. a shape and configurationof the outer wall facilitating gripping of the housing with a human handB. an acoustic pipe for transmitting acoustical energy and receiving thereflected acoustical energy, mounted in the aperture said pipe having afirst end within the chamber and an open second end outside of thehousing, said second end of the acoustic pipe being adapted forconnection of the acoustic pipe to an orifice leading into therespiratory tract; C. a lauching transducer mounted in the chamber andcoupled to the first end of the acoustic tube, for launching acousticalenergy into the acoustic pipe, propagating and incident wave out of theopen second end; D. at least one acoustic pressure wave sensingtransducer mounted on the acoustic pipe at a location between the firstand second ends of the acoustic pipe for sensing reflections of theincident wave, received back in the acoustic tube through the opensecond end and generating a signal; and E. means at least partiallywithin the chamber, connected to the acoustic wave sensing transducer,to processor means for processing said echo signals into an acousticimage signal characteristic of the morphology of a site within themammal's respiratory tract said coupler comprising:(a) a tube having afirst end adapted by size and configuration to sealingly mate with saidorifice; (b) a second end adapted by size and configuration to bemounted on the acoustic pipe of the head assembly; (c) a tube bodyextending between the first and second ends of the tube, said bodyhaving an internal contour such that impedance matching between the headassembly and the mammal is maintained with maximum acoustic energytransmission; said head assembly comprising an assembly for acousticallyimaging portions of the internal morphology of the respiratory tract ofa mammal, including a human, which comprises a lightweight, easy tomanipulate, hand-held acoustic imaging head which is rugged and entirelyhand supportable and operable by an operator, throughout an imagingprocedure.
 2. The coupler of claim 1 wherein the body tapers inwardlyfrom the second end to the first end at an angle of about 10° to 15°from the perpendicular.
 3. The coupler of claim 1 wherein the first endis adapted to seal with the nostril opening of said mammal.
 4. Thecoupler of claim 1 wherein the tube body is fabricated from a flexiblematerial.
 5. The coupler of claim 4 wherein said material is apolysiloxane rubber.
 6. The coupler of claim 1 wherein the thickness ofthe body is greater proximal to the second end than the thicknessproximal to the first end.
 7. The coupler of claim 1 wherein the firstend is circular.
 8. The coupler of claim 1 which may be trimmed proximalto the first end to obtain a larger size opening at the first end. 9.The coupler of claim 1 which is disposable.